Wireless cellular communication networks incorporate a number of mobile user equipments (UEs) and a number of NodeBs. A NodeB is generally a fixed station and may also be called a base transceiver system (BTS), an access point (AP), a base station (BS) or some other equivalent terminology. NodeB functionality evolves as improvements of networks are made, so a NodeB is sometimes also referred to as an evolved NodeB (eNB). In general NodeB hardware is fixed and stationary. In contrast user equipment hardware is generally portable. User equipment, commonly known as a terminal or a mobile station, may be fixed or mobile device and may be a wireless device, a cellular phone, a personal digital assistant (PDA) or a wireless modem card. Uplink (UL) communication refers to a communication from the mobile user equipment to the NodeB. Downlink (DL) communication refers to communication from the NodeB to the mobile user equipment. Each NodeB contains radio frequency transmitters and receivers used to communicate directly with plural mobiles, which move freely around it. Similarly, each mobile user equipment contains a radio frequency transmitter and a receiver used to communicate directly with the NodeB. In cellular networks, the mobiles cannot communicate directly with each other but must communicate with the NodeB. The coverage area of a NodeB is generally split into multiple serving cells or sectors.
Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access Network (E-UTRAN), are being standardized by the 3GPP working groups (WG). Orthogonal frequency division multiple access (OFDMA) was chosen for the downlink (DL) of E-UTRAN and single carrier frequency division multiple access (SC-FDMA) was chosen for uplink (UL) of E-UTRAN. OFDMA and SC-FDMA symbols are hereafter referred to as OFDM symbol. User equipments are time and frequency multiplexed on a physical uplink shared channel (PUSCH) and a fine time and frequency synchronization between user equipment guarantees optimal intra-cell orthogonality. The user equipment autonomously maintains its DL synchronization from DL synchronization signals broadcast by the base station. UL synchronization requires base station involvement. In case the user equipment is not UL synchronized, it uses a non-synchronized Random Access Channel (RACH). The base station provides back some allocated UL resource and timing advance information to permit the user equipment to transmit on the PUSCH.
Orthogonal frequency division multiple access (OFDMA) is a multi-user version of orthogonal frequency division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of sub-carriers carriers to individual user equipment. This allows simultaneous low data rate transmission from several users. Based on feedback information about the channel conditions, adaptive user-to-sub-carrier assignment can be achieved. If the assignment is done sufficiently fast, this further improves the OFDM robustness to fast fading and narrow-band co-channel interference and makes it possible to achieve even better system spectral efficiency. Different number of sub-carriers can be assigned to different users to support differentiated quality of service (QoS). This controls the data rate and error probability individually for each user.
Control information bits are transmitted in the uplink (UL) for several purposes. For example, a downlink hybrid automatic repeat request (HARQ) requires at least one bit of ACK/NACK transmitted information in the uplink indicating successful or failed circular redundancy checks (CRC). Furthermore, an indicator of downlink channel quality (CQI) needs to be transmitted in the uplink to support mobile user equipment scheduling in the downlink. While CQI may be transmitted based on a periodic or triggered mechanism, the ACK/NACK needs to be transmitted in a timely manner to support the HARQ operation. Note that ACK/NACK is sometimes denoted as ACKNAK or just simply ACK or other equivalent term. In this example, some elements of the control information should be provided additional protection compared with other information. For instance, the ACK/NACK information is typically required to be highly reliable in order to support appropriate and accurate HARQ operation. This uplink control information is typically transmitted using the physical uplink control channel (PUCCH), as defined by the 3GPP working groups (WG), for evolved universal terrestrial radio access (E-UTRA). The E-UTRA is sometimes also referred to as 3GPP long-term evolution (3GPP LTE). The structure of the PUCCH is designed to provide sufficiently high transmission reliability.
In addition to PUCCH, the E-UTRA standard also defines a physical uplink shared channel (PUSCH) intended for transmission of uplink user data. The physical uplink shared channel (PUSCH) can be dynamically scheduled. Thus the time and frequency resources of PUSCH are re-allocated every sub-frame. This reallocation is communicated to the mobile user equipment using the physical downlink control channel (PDCCH). Alternatively, resources of the PUSCH can be allocated semi-statically via a mechanism called persistent scheduling. Thus, any given time and frequency PUSCH resource can possibly be used by any mobile user equipment depending on the scheduler allocation. Physical uplink control channel (PUCCH) is different than the PUSCH. The PUCCH is used for transmission of uplink control information (UCI). Frequency resources which are allocated for PUCCH are found at the two extreme edges of the uplink frequency spectrum. In contrast, frequency resources which are used for PUSCH are in between. Because PUSCH is designed for transmission of user data re-transmissions are possible. The PUSCH is expected to be generally scheduled with less stand-alone sub-frame reliability than the PUCCH. The general operation of the physical channels are described in the E-UTRA specifications, for example: “3GPP TS 36.3211 v8.2.0, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8).” The present invention is described in the context of the E-UTRA wireless network, but applies as well to other asynchronous networks implementing hard handovers.
As the user equipment moves in the network, it will need to handover from a base station of a serving cell to a base station of a target cell. The base station of the target cell is selected to be a better cell at that particular time and location for that user equipment. In cellular networks where the user equipment can be served by multiple base stations simultaneously, the user equipment typically performs a soft handover when transitioning from a cell to another. This means that the user equipment will be served by both base stations during a period of time before finally detaching from the first base station. This has the benefit of providing no interruption time, but requires some complexity which impacts the cost of both user equipment and base station. In cellular networks where the user equipment can be served by only one base station at a time, the user equipment typically performs a hard handover when transitioning from one cell to another. This means that the user equipment first detaches from the first base station before accessing the target base station. This results in some unavoidable interruption time.
A handover process generally consists of two procedures. The first procedure occurs between the two base stations and is called context transfer. The serving base station negotiates user equipment access to the target base station. This includes passing along through a backhaul access all necessary user equipment context information for the target base station to provide a continuous service to the user equipment. The second procedure happens at the user equipment. This second procedure consists of synchronizing and accessing to the target base station from the user equipment. Handover (HO) interruption time in asynchronous wireless networks can be attributed significantly to the process of UL synchronization after breaking from the source cell. The E-UTRA RACH process, especially for the contention based RACH involves significant latencies. The handover latencies need to be minimized in an efficient manner.
For simplicity handover means intra-LTE to inter-base station handover in LTE_ACTIVE. Handover latency begins as soon as the source base station stops transmissions to the user equipment and ends when the first UL message that can potentially carry useful data is transmitted by the target base station. The main cause for handover interruption time is the user equipment must complete any final formalities with the source base station and achieve UL synchronization and initial allocation with the target base station. In synchronous networks, which are networks in which the base stations know the relative time difference between them, the timing advance (TA) in the target base station can be computed autonomously without any initial RACH transmission in the UL of the target base station. However, in asynchronous networks an initial RACH access in the target base station is required for the target base station to compute the TA and forward it to the user equipment in some way.